Cosmic rays are charged particles that rain down on Earth from space, with energies that can reach as high as a fast-thrown baseball packed into a single subatomic particle. Although a lot is known about cosmic rays, their origin still remains a century-old mystery. The highest energy particles, called ultra-high-energy cosmic rays (UHECRs), are closely linked to extremely high-energy (EHE) neutrinos, weakly interacting particles that are able to travel undeflected through the cosmos. The IceCube Neutrino Observatory at the South Pole searches for the sources of these EHE neutrinos in order to understand how particles like cosmic rays are accelerated to the highest known energies in the universe.
Since IceCube was fully funded in 2004, a research group led by Professors Shigeru Yoshida and Aya Ishihara at the International Center for Hadron Astrophysics at Chiba University in Japan has searched for EHE neutrinos. The group established an analysis method that can reliably detect any type of neutrino if they are at these ultrahigh energies. Using this method, IceCube observed two ultra-high-energy neutrino events in 2012 that were the first of their kind ever observed and the most energetic neutrino events detected at the time. This was the initial indication that cosmic neutrinos may be produced by very high-energy cosmic rays.
“When we started the EHE neutrino search program, nobody in the particle astrophysics community thought that IceCube would perform well in such a high-energy region,” says Yoshida. “But we were pretty confident we could do it. To me, the two ultra-high-energy neutrinos we observed were like a stamp of approval for our long-standing belief.”
Now, after a 20-year effort, a new search for EHE neutrinos using 13 years of IceCube data has resulted in the most stringent upper limit on the EHE neutrino flux to date. The results also provide compelling evidence that rejects the prevailing theory of UHECRs being proton dominated. Rather, IceCube’s results suggest that the primary component of UHECRs is heavier nuclei.
The study, which was co-led by Chiba University assistant professor Maximilian Meier and University of Maryland assistant professor Brian Clark, will be published in the July edition of Physical Review Letters, with the paper being selected as an Editors’ Suggestion, a recognition of its significant contributions to the field. A press conference summarizing the results will also be held on Friday, July 11 at 2:00 PM JST at the Ministry of Education, Culture, Sports, Science and Technology in Toranomon, Tokyo, Japan.
“EHE neutrinos must exist if cosmic rays are protons,” says Ishihara. “The absence of a signal still sets an upper limit on the proton contribution, which provides valuable information in understanding the nature of UHECR production sites.”
The findings also raise significant questions about the KM3NeT experiment’s report in Nature of the detection of a neutrino with an energy of 220 PeV, equivalent to approximately 100 quadrillion that of visible light. Given that IceCube has been running longer and boasts a sensitivity that is 70 times greater than that of KM3NeT, the underlying reason for the results remains a mystery.
“The KM3NeT event may point at some unexpected science in the energy region where IceCube runs out of statistics, or they were just very lucky,” says Francis Halzen, the principal investigator of IceCube. “It is part of statistics that rare events happen.”
The fact that current kilometer-scale neutrino observatories have detected only a single event over 100 PeV after more than a decade suggests that the statistical uncertainties are still very high, calling for larger detectors that can gather more data. For IceCube, that is the proposed IceCube-Gen2, which will be about eight times larger and eight times more sensitive than the current iteration.
“Although our 20-year journey has brought us to a place where we can say meaningful things about UHECRs and neutrinos, we have yet to find super high-energy neutrinos,” says Yoshida. “With IceCube-Gen2, we will be able to measure more neutrinos and get closer to revealing the origin of UHECRs.”